Alonso entered the 2009 season ranked as the number one overall prospect in the Reds organization by Baseball America.[6] He started the season with Sarasota, played 49 games, and hit .303 in 175 at-bats with seven home runs, 13 doubles, and 38 RBIs. After receiving a promotion to Class AACarolina, he hit .295 in 105 at-bats, with 11 doubles, two homers, and 14 RBIs over 29 games.[7] He also played for the Peoria Saguaros of the Arizona Fall League in 2009, .267 in 23 games.[8]

Alonso started 2010 in Carolina playing left field, as he is blocked at first base in Cincinnati by Joey Votto.[9] Alonso was called up by the Reds on September 1, with rosters expanding.[10]

Alonso made his debut with the big club on September 1st 2010, pinch hitting for Brandon Phillips. On September 4, 2010, in his second at bat as a pinch hitter, Alsonso got his first major league hit, a double off St. Louis Cardinals pitcher Mike MacDougal. Later in the inning, he scored his first major league run.